1. Field of the Invention
The present invention relates to a process for producing cellulose acetate. In particular, the present invention relates to a process for producing cellulose acetate which is particularly useful when a low-grade wood pulp sheet having a high sheet density is employed as a starting material. The disintegration is conducted in the presence of a small amount of water fed into the disintegrating section of a disintegrator during the step of disintegrating the starting pulp sheet.
2. Description of the Related Art
Cellulose acetate is an organic acid ester of cellulose and is used for various purposes such as the material of fibers for clothes, cigarette filters, chips plastics, films and paints. The production of cellulose acetate is the largest of the cellulose derivatives. Cellulose acetate is thus industrially important.
A typical example of the processes for producing secondary cellulose acetate on an Industrial scale is the so-called acetic acid process wherein acetic anhydride is used as an acetylating agent, acetic acid is used as a diluent and sulfuric acid is used as a catalyst.
The acetic acid process for producing cellulose acetate is outlined as follows: this process comprises (1) a pretreatment and activation step wherein a cellulose material having a relatively high .alpha.-cellulose content is disaggregated and disintegrated, and acetic acid, which may contain a small amount of an acid catalyst, is sprayed thereon and mixed therewith, (2) an acetylation step wherein the activated cellulose obtained in step (1) is treated with an acid mixture comprising acetic anhydride, acetic acid and an acid catalyst (such as sulfuric acid) to obtain a primary cellulose acetate, (3) an aging step wherein the primary cellulose acetate is hydrolyzed to obtain a secondary cellulose acetate having a desired degree of acetylation, and (4) an aftertreatment step wherein the secondary cellulose acetate thus obtained is separated from the reaction solution by precipitation, purified, stabilized and dried.
Techniques of improving the above-described basic steps developed from various viewpoints were proposed. The present invention relates to a process for producing cellulose acetate, especially from a low-grade wood pulp. The techniques proposed heretofore in this technical field will be described below. "A low-grade pulp" has an .alpha.-cellulose content of about 98% or below, generally.
It has been known that secondary cellulose acetate having excellent transparency, filterability and spinnability is produced by disintegrating a wood pulp having an .alpha.-cellulose content of 92 to 93% in a dilute aqueous acetic acid solution to obtain a slurry and then repeating dehydration and substitution with acetic acid to conduct a so-called slurry pretreatment, preparing the primary cellulose acetate by an ordinary method, completely neutralizing the sulfuric acid catalyst in the reaction system, and saponifying and aging the reaction mixture at 125.degree. to 170.degree. C.
It has also been known that secondary cellulose acetate having excellent transparency, filterability and spinnability is produced by disintegrating a high-grade pulp having a high .alpha.-cellulose content by an ordinary dry method, conducting acetylation in the presence of a small amount of sulfuric acid as the catalyst at a high temperature to complete the acetylation in a short time and to obtain primary cellulose acetate, completely neutralizing the sulfuric acid catalyst in the reaction system, and conducting saponification and aging at 110.degree. to 120.degree. C. Even a low-grade pulp having a low .alpha.-cellulose content can be used as a starting material in this process.
The above-described processes are characterized in that the saponification and aging are conducted at a high temperature. By the high-temperature saponification and aging, the breakage and change in the shape of acetylated hemicellulose harmful to the properties of cellulose acetate are accelerated to make it harmless.
High-grade pulp having a high .alpha.-cellulose content has been used heretofore as the starting cellulose for cellulose acetate.
It is generally conceivable that cellulose materials having a high grade and a reasonable price or cost are not generally available on the market, because of the worldwide limitation of resources and environmental pollution caused by pulp production plants. However, the above-mentioned typical example of processes for producing secondary cellulose acetate on an industrial scale, that is, the so-called acetic acid process, and the process using a high-grade pulp having a high .alpha.-cellulose content described above are disadvantageous when a low-grade pulp having a low .alpha.-cellulose content is employed as the starting cellulose.
Namely, pulp sheets prepared from the low-grade pulp having a low .alpha.-cellulose content are hard because of the essential uses thereof and, therefore, they cannot be disaggregated or disintegrated to an extent suitable for the pretreatment and activation or acetylation in a dry pulp disintegrator widely used in ordinary cellulose acetate producing techniques. As a result, a large amount of unreacted fibers or a gel formed by the incomplete reaction are contained in the acetylation product and, therefore, it cannot be used as the final product.
Although the above-described technique of producing cellulose acetate from a low-grade pulp having a low .alpha.-cellulose content is preferred under these circumstances, it still has industrial defects in practice.
Namely, although the above-described problem can be solved by the slurry pretreatment process, a large amount of a dilute aqueous acetic acid solution formed in the step must be concentrated and recovered. It is thus difficult to employ this process for the production on an industrial scale from the economic viewpoint.
European Patent Publication-A2 No. 351226 (published on Jan. 17, 1990) and the corresponding Japanese Patent Laid-Open No. 91101/1990 (published on Mar. 30, 1990) disclose a process wherein a pulp sheet is impregnated with water prior to disintegration to previously soften it so as to prevent the reactivity of the pulp from lowering in the disintegration step. However, in these processes wherein the whole pulp sheet is impregnated with water, a large quantity of water is necessitated and, therefore, the drying cost for removing excess water after the completion of the disintegration is high. Thus, this process cannot be easily employed from the economic viewpoint.